Ceramic membranes are most commonly used to separate mixtures of liquids and gases, and to catalyze chemical reactions. The membranes are advantageous because both membrane composition and pore size may be adjusted to produce membranes having the properties desired for the particular separation or catalysis. Much effort has, therefore, been directed to improving the catalytic efficiency and throughput of ceramic membranes, particularly in membranes used both for catalytic dehydrogenation and for separating the dehydrogenation products.
Membranes having smaller pores are more useful for separations on the basis of size exclusion. Methods exist for synthesizing membranes having a desired porosity by varying the size of particles in the colloidal sol from which the membrane is formed, as detailed below.
Anderson, et al., J. Memb. Sci 39: 243-258 (1988), describes various methods of making particulate sols and membranes from transition metal oxides. Particulate ceramic membranes are typically formed through a process beginning with metal-organic compounds. The compounds are hydrolyzed to form small metal oxide clusters, which in turn aggregate to form metal oxide particles. The particles are then fused into a unitary ceramic material. The gaps between the fused particles form a series of pores in the membrane.
Metal oxide ceramic membranes are generally created using a sol-gel procedure. Usually, a metal oxide is initiated into the process as a metal alkoxide solution. The metal alkoxide is hydrolyzed to form metal hydroxide monomers, clusters or particles, depending on the quantity of solvent used. The insoluble metal oxide particles are then peptized by the addition of an acid which increases the tendency of the metal oxide particles to remain in suspension, presumably due to charges acquired by the particles during the peptizing process.
Such a sol can be evaporated to form a semi-solid gel. Further evaporation, and then sintering, of the gel results in a durable rigid material formed either as an unsupported membrane or as a supported membrane or thin film coated onto a substrate. The substrate can be either porous or non-porous, and either metallic or non-metallic, depending on the particular application.
With regard to the composition of the membranes, ceramic membranes have been created using many materials. For example, Leenaars et al., Jour. of Membrane Science, 24: 261-270 (1985), report the use of the sol-gel procedure to prepare supported and unsupported alumina membranes. In addition, Yoldas conducted significant research on the fabrication of gamma-alumina membranes made by a sol-gel process. Yoldas was able to achieve a relatively small particle size in the sols and was able to achieve porous membranes. Yoldas, Jour. Mat. Sci., 12:6, pp. 1203-1208 (1977). Yoldas also investigated the manufacture, through sol-gel processes, of mixed alumina and silica materials. One class of materials which Yoldas prepared were alumina-siloxane derivatives which formed polymeric cross-linkages making polymeric, rather than particulate, ceramic materials. Yoldas was also able to make several large monolithic glass samples of varying compositions of silica and alumina that did retain some porosity and high surface area, as described in the article in Jour. Mat. Sci., supra. Ceramic membranes composed of titania, zirconium and other metal oxides have also been reported.
The mechanism of operation and types of separations which can be achieved by ceramic membranes are discussed in general by Asaeda et al., Jour. of Chem. Enq. of Japan, 19[1]: 72-77 (1986). At least one line of ceramic filters is currently marketed under the trade name "Ceraflo" by Millipore Corporation of Worcester, Mass.
Ceramic membranes offer several advantages over organic membranes, which are also used for separation and catalysis. Ceramic membranes are more resistant than organic membranes to organic solvents, chlorine, and extremes of pH. Ceramic membranes are also inherently more stable than organic membranes at high temperatures, thus allowing more efficient sterilization of process equipment. Ceramic membranes are also generally quite resistant to microbial or biological degradation, which can occasionally be a problem with organic membranes. Ceramic membranes are also more mechanically stable under high pressures.
Although the pore size and composition requirements of ceramic membranes have been extensively researched, the art is not similarly developed with respect to macroscopic membrane attributes that may be optimized to further enhance the utility of ceramic membranes for separation and catalysis.